Comey confirmed that under our Constitution, the president has the authority to direct the FBI to stop investigating any individual. I paraphrase, because the transcript is not yet available: the president can, in theory, decide who to investigate, who to stop investigating, who to prosecute and who not to prosecute. The president is the head of the unified executive branch of government, and the Justice Department and the FBI work under him and he may order them to do what he wishes.